My detached garage has water and power nearby. What does conversion to a legal AADU run?

Question

Marcus Golen · Accepted Answer

Detached garage-to-DADU conversion in Seattle typically runs $220–$340k for 400–600 sqft. The garage shell saves you ~$40–60k versus ground-up but you almost always need new foundation work (most garages don’t meet residential code), full insulation, new electrical service, water/sewer extensions, and full Energy Code compliance (R-21 walls, R-49 roof). Cheapest path: keep the existing footprint, avoid second-story additions, and run utilities along the existing trench.

Answer

For a fully detached, code-built DADU in Seattle (2026), realistic all-in pricing is $340k–$520k for 600–1,000 sqft. That includes design ($18–35k), SDCI permit fees ($14–22k), site work and utility runs ($30–70k), construction ($240–380k), and a 10–15% contingency. Quotes under $250k almost always exclude utility sepa

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AADU wins on upfront cost (typically $180–$300k for a basement conversion) but rents for 15–25% less and is harder to refinance separately. DADU costs more ($340–$520k) but commands premium rent ($2,400–$3,400/mo in Seattle), is easier to refinance with separate utility meters, and adds more resale value. ROI math: a D

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In 2026, a new 600–800 sqft DADU in core Seattle (Ballard, Capitol Hill, Wallingford, West Seattle) rents for $2,400–$3,200/mo unfurnished and $3,200–$4,500/mo furnished/short-term. North Seattle (Lake City, Northgate) is 10–15% lower. Outside Seattle, expect Bellevue $2,600–$3,400, Kirkland $2,400–$3,100, Redmond $2,5

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Recent (2024–2026) Seattle appraisal data shows DADUs add 65–85% of build cost to appraised value at first appraisal — so a $400k DADU adds roughly $260–340k. Within 3–5 years, as comps mature, that ratio rises to 85–110%. AADUs add less in absolute dollars but a higher ratio (~90% of cost) because they don’t require n

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For a custom DADU in Seattle, design + structural + civil + energy modeling typically runs $22–38k (roughly 6–8% of construction cost). Pre-approved plans drop that to $8–14k (site adaptation only). Beware quotes under $15k for custom — they usually exclude structural calcs, energy compliance, or revisions, all of whic

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Expect your property tax bill to increase by $3,500–$6,500/year for a typical $400k DADU in Seattle (King County’s effective rate is roughly 0.85–1.05%). That’s a real expense to underwrite — at $5k/year, it eats ~14% of typical DADU rental income. Reassessment usually happens 6–18 months after final inspection. Plan f

What does it cost to convert a garage into an AADU in Seattle?

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